The present invention relates to a titanium target assembly for sputtering, which has a solid phase diffusion joining (bonding) plane, and a method for preparing the same and, in particular, to a titanium target assembly for sputtering obtained by integrating a titanium (Ti) target, which constitutes a cathode used in a thin film-forming sputtering equipment, and a backing plate serving as a support for the target through a coating metal film according to the solid phase diffusion joining (bonding) technique as well as a method for preparing the same.
The sputtering target may serve as a sputtering source for forming a thin film on a substrate in the production of a variety of thin film devices including a semiconductor device. In the case of semiconductor devices, most of the targets are plates each having a disk-like shape. The sputtering target currently used is in the form of a target assembly in which a target is integrated with a support member called a backing plate used for supporting and cooling the target. When a target assembly is fitted to a sputtering cathode and then an electric power is applied to the cathode, the temperature of the target surface is raised due to the application of heat, but the heat is removed from the target by passing cooling water through the back face of the backing plate to thus control any increase of the temperature of the target surface. Materials for the backing plate commonly used are, for instance, metals and alloys thereof such as oxygen free copper, copper alloys, aluminum, aluminum alloys, titanium and titanium alloys.
Conventionally, the target and the backing plate have principally been joined through the use of a low melting brazing filler metal such as an In or Sn alloy. When a high electric power is applied for the purpose of the improvement of the throughput, however, the temperature at the interface to be joined inevitably increases despite of the cooling of the backing plate side and various problems or troubles arise such that the bonding strength is reduced due to the increase in the temperature and that the target material is in turn peeled off due to the melting of the brazing filler metal. For this reason, the joining method has been switched over to the integrated joining method, in which any brazing filler metal is not used, depending on the sputtering target. When a higher electric power is applied to the cathode for the further improvement of the throughput, however, the target undergoes changes in the characteristic properties due to the heat. Moreover, the size of the target assembly has been increased as the size of a wafer increases. This leads to the occurrence of considerable warpage of the target assembly as a joined body of different materials when such a high electric power is applied because of the difference in the coefficient of thermal expansion between these materials for the target assembly and this in turn becomes a cause of inconvenience such as the unevenness of the film thickness and changes in the film quality. Therefore, there has been desired for the appropriate selection of materials for the backing plate, while taking into consideration the control of the cooling efficiency and the warpage.
On the other hand, it has been desired for the target/backing plate assembly to satisfy such a requirement that xe2x80x9cthe crystalline structure and crystalline orientation of the target material should be maintained and the strength of the backing plate material should be ensuredxe2x80x9d and that xe2x80x9cany deformation of the target material and the backing plate material should be controlled by reducing a joining loadxe2x80x9d upon the junction of these materials for the assembly, in addition to the foregoing control of the cooling efficiency and the warpage.
In respect of the joining method, in which any brazing filler metal is not used and which is used for producing the target/backing plate assembly, there have been filed a large number of patent applications. For instance, Japanese Un-Examined Patent Publication (hereunder referred to as xe2x80x9cJ. P. KOKAIxe2x80x9d) No. Hei 1-283367 (Japanese Patent No. 2,831,356) discloses that the target and the backing plate are directly welded under pressure without the interposition of any bonding material and lists up an explosive pressure welding technique and a seam welding technique as embodiments thereof. This patent also discloses the results obtained by the hot-rolling and diffusion joining or bonding techniques and states that good bonding strength could be achieved by these techniques.
As another direct joining method, J. P. KOKAI No. Hei 6-65733 discloses a diffusion bonding method comprising the steps of pre-heating a support member and a target member, laying one on top of the other and applying a load to these members by pressing.
As still another direct joining method, J. P. KOKAI No. Hei 9-143704 discloses a diffusion bonding method comprising the steps of bringing a titanium target material into contact with an aluminum backing plate material and then subjecting these materials to hydraulic pressing at temperatures ranging from 300 to 450xc2x0 C. and pressures ranging from 50 to 200 MPa. This publication also discloses, in Examples, that a bonding strength on the order of 91 MPa (9.3 kg/mm2) could be obtained under the conditions of 450xc2x0 C. and 120 MPa and that a bonding strength on the order of 35 MPa (3.6 kg/mm2) could be obtained under the conditions of 450xc2x0 C. and 45 MPa.
As has been discussed above, there have been filed a large number of patent applications concerning the direct bonding method, but the majority thereof is limited to the combination of Ti/Al and the bonding strength of the assemblies achieved by these methods widely vary.
The inventors of the present invention applied the diffusion bonding method disclosed in J. P. KOKAI No. Hei 1-283367 (Japanese Patent No. 2,831,356), which is not seriously limited in the material for the target assembly among other techniques, to the combination of Ti (target)/Cu (backing plate) and carried out bonding tests. As a result, it was found that the bonding method provided results lacking in bonding stability and reliability and showing high scattering and more specifically, the bonding strength was found to fall within the range of from 6 to 11 kg/mm2 under the conditions of a bonding temperature of 500 or 450xc2x0 C. and a pressure of 40 MPa.
The inventors of this invention also attempted to directly bond the combination of Ti/Cu under the conditions of 400xc2x0 C. and 100 MPa, which were selected on the basis of those disclosed in the other conventional techniques relating to the Ti/Al bonding. In any case, however, the bonding strength attained are low on the order of 1.5 kg/mm2 and accordingly, this method never ensures the desired or required bonding stability and reliability at all.
On the other hand, as bonding methods, which make use of an insert material for the purpose of the reduction of the load to be applied or the reduction of the bonding temperature, there have been disclosed methods for obtaining xe2x80x9ctarget assemblies each comprising a target material, an insert material and a backing plate material having a solid phase diffusion bonding interface between each neighboring two materialsxe2x80x9d, in J. P. KOKAI Nos. Hei 6-108246 and Hei 6-172993. More specifically, these publications disclose that a desired assembly can be prepared by solid phase diffusion-bonding the target and backing plate materials through an insert material consisting of Ag, Cu and/or Ni at a temperature falling within the range of from 200 to 600xc2x0 C. and an applied load falling within the range of from 0.1 to 20 kg/mm2. In Examples of these publications, a bonding strength (or shearing strength) of 6 kg/mm2 can be accomplished by bonding a Ti target material and an oxygen free copper backing plate material (J. P. KOKAI No. Hei 6-108246) or an Al alloy target material and an oxygen free copper backing plate material (J. P. KOKAI No. Hei 6-172993), using an Ag foil having a thickness of 100 xcexcm as an insert material at a temperature of 250xc2x0 C. and an applied load of 8 kg/mm2. When the inventors of the present invention have conducted a test for bonding Ti and Cu using an Ag foil as an insert material according to the foregoing method, however, the resulting bonding strength is lacking in the bonding stability and reliability. More specifically, the strength widely varies within the range of from 3 to 7 kg/mm2 at a temperature of 500xc2x0 C. and a load of 4 kg/mm2 and the bonding operation at a temperature of 450xc2x0 C. and a load of 4 kg/mm2 simply achieves such a strength that a sample for evaluating the bonding strength is broken during the production thereof.
As has been described above in detail, there have been known a large number of conventional techniques concerning the bonding of a target and a backing plate. If the inventors of the present invention conducted tests of bonding Ti (target) and Cu (backing plate) according to the conditions disclosed in these conventional techniques, however, it was found that the resulting assemblies did not show any high bonding strength, that they showed broad bonding strength distributions and that they were insufficient in the bonding stability and reliability.
The inventors of the present invention have thus variously investigated such results and have concluded that the following two requirements should be satisfied to obtain a target assembly showing a high bonding strength and high bonding stability and reliability:
(i) A continuum or a continued body should be formed between different metallic materials; and
(ii) The formation of an intermetallic compound between the different metallic materials should be controlled.
Regarding the foregoing item (i), these different metals never undergo any mutual diffusion or interdiffusion therebetween if the metals each has not a mutual solid solubility, any continuum cannot be formed unless they cause any mutual diffusion and they are simply and mechanically bonded together. On the other hand, in respect of the foregoing item (ii), if an intermetallic compound is formed between them, the compound in itself serves as a stress concentration point since it is quite hard and brittle by nature and accordingly, the bonding stability and reliability of the resulting assembly is considerably reduced.
In fact, the results obtained by the observation of the broken-out section in the bonding test carried out according to the desired conditions indicate in conventional techniques that the metals do not undergo any sufficient mutual diffusion or that an intermetallic compound is formed.
From the foregoing, it would be concluded that the following independent and reciprocal requirements should be satisfied in order to obtain a target assembly possessing a high bonding strength and high bonding stability and reliability:
(a) Selection of materials each having a mutual solid solubility;
(b) Acceleration of mutual diffusion by a cleaning treatment of the interface; and
(c) Inhibition of the formation of an intermetallic compound by the control of the processing temperature and time.
In particular, the acceleration of the mutual diffusion (b) is an essential requirement to relieve the surface barrier against the bonding and diffusion between the metals, among others.
On the other hand, in the target/backing plate assembly, it is desired, by nature, that the crystalline structure and crystalline orientation of the target material should be maintained and the strength of the backing plate material should likewise be ensured and further it is also desired that any deformation of the target material and the backing plate material should be controlled by reducing a joining load upon the junction of these materials. Accordingly, to obtain a desired target assembly, it would be desired to develop a novel joining or bonding method, which can simultaneously satisfy these requirements.
Among the foregoing conventional techniques, the invention disclosed in J. P. KOKAI No. Hei 1-283367 (Japanese Patent No. 2,831,356) suffers from a problem in that the bonding strength of the resulting target assembly is rapidly reduced when the bonding temperature is lowered, the inventions disclosed in J. P. KOKAI Nos. Hei 6-108246 and Hei 6-172993 suffer from such a problem that the bonding strength of the resulting assembly widely varies due to the incorporation of an insert material and the invention disclosed in J. P. KOKAI No. Hei 9-143704 suffers from such a problem that the materials undergo deformation by the application of a high load and the resulting bonding strength is not sufficiently high.
Moreover, the foregoing conventional techniques simply disclose the bonding strength of the resulting assemblies and these techniques never refer to the extent of the contribution of the diffusion at the boundary to the bonding or junction between the materials. It is desirable to conduct the diffusion bonding at a higher temperature in order to induce such a change in the broken boundary (due to the diffusion at the boundary) that the resulting assembly shows the bonding stability and reliability, but the bonding temperature should be reduced in order to satisfy the foregoing requirements: xe2x80x9cinhibition of the formation of an intermetallic compoundxe2x80x9d and xe2x80x9cthe crystalline structure and crystalline orientation of the target material should be maintained and the strength of the backing plate material should likewise be ensuredxe2x80x9d. In other words, reciprocal two requirements should be satisfied.
Accordingly, it is an object of the present invention to solve the foregoing problems associated with the conventional techniques in a broad sense and more specifically to provide a titanium target assembly for sputtering having a high bonding strength and high bonding stability and reliability as well as a method for preparing such an assembly according to the solid phase diffusion bonding at a low temperature and a low applied load for a short period of time.
The inventors of this invention had completed a bonding method, which can ensure sufficient solid phase diffusion at the bonding interface even under the following conditions: in a vacuum, at a low temperature and a low bonding load and for a short period of time by depositing an active metal film on the surface of the target material on the side to be bonded according to the physical vapor deposition technique to thus cover the surface and already filed a patent application: Japanese Patent Application Serial No. 2000-72380 (filing date: Heisei 12 Mar. 15, 2000.
The titanium target assembly for sputtering and the method for preparing the same according to the present invention have been completed by expansively developing the invention disclosed in the foregoing earlier filed patent application.
According to an aspect of the present invention, there is provided a titanium target assembly, which comprises a sputtering target composed of high purity titanium, a backing plate composed of copper or a copper alloy and serving as a support member for the target and a coating film composed of silver or a silver alloy and formed and sandwiched between the target and the backing plate, wherein the coating film is formed on the surface subjected to a cleaning treatment on the bonding side of the target or on the bonding sides of the target and the backing plate according to the physical vapor deposition technique and the titanium target and the backing plate are solid phase diffusion bonded, while the face(s) provided with the coating film serves as the bonding plane.
According to another aspect of the present invention, there is provided a method for preparing a titanium target assembly for sputtering by bonding a sputtering target composed of high purity titanium and a backing plate composed of copper or a copper alloy and serving as a support member for the target to thus give a titanium target assembly for sputtering and the method comprises the steps of (1) treating the surface of the target and/or the backing plate on the bonding side(s) thereof with inert gas plasma according to the sputter etching technique or the ion bombardment technique to thus make the bonding surface(s) clean, (2) forming a coating film composed of silver or a silver alloy on the cleaned bonding surface(s) by the physical vapor deposition (PVD) technique without opening to the atmosphere and immediately after the completion of the cleaning step and (3) solid phase diffusion-bonding the target and the backing plate through the surface(s) provided with the coating film to thus form a titanium target assembly having a solid phase diffusion bonding plane.